Various vapor deposition processes are known to be useful, for example, in the production of semiconductor products. These processes are typically used to deposit very thin layers of various substances including conductive, semiconductive and insulative materials onto a substrate. Vapor deposition processes typically require each deposited material to be transported to the deposition chamber in a gas state or vapor phase where it is condensed onto the work in process. Efficient operation of such a deposition process requires precise control of the pressure of the gases or vapors used in the process. Where the deposit material in its vapor phase has a relatively low condensation temperature (i.e., well below room temperature) the pressure of the material may be controlled using pressure transducers operating at room temperature. However, where the gas state or vapor phase of a deposit material has a relatively high condensation temperature, i.e., above room temperature, to avoid condensation, such materials are heated and maintained above their condensation temperatures, and thus heated transducers are usually required for measuring the pressures of these hot gases and vapors. Heated pressure transducers are also often heated to prevent sublimation or precipitation of solid material. For example, as is well known, ammonium chloride (NH.sub.4 Cl) is a chemical by-product of processes for depositing layers of silicon nitride (Si.sub.3 N.sub.4), and if the pressure and temperature drop too low the NH.sub.4 Cl sublimates so that a solid salt forms on any exposed cool surfaces. To prevent such sublimation of NH.sub.4 Cl, these processes are often conducted at 150.degree. C.
FIG. 1 shows a sectional view of a portion of a prior art heated pressure transducer assembly 100 of the type that is typically used in connection with relatively high temperature, vapor deposition processes. Transducer assembly 100 includes a pressure sensitive sensor 110 that is housed within an interior cavity defined by an external enclosure or housing 112. Sensor 110 is of the capacitive type and includes an input port 110a for receiving the heated vaporized material and two output terminals (not shown) for providing an electrical signal representative of the pressure of the vaporized material entering the transducer assembly. The sensor 110 is configured with a capacitive element so that the measured capacitance created between the two output terminals varies according to a function of the pressure at input port 110a.
Transducer assembly 100 further includes a tube 114 for coupling a source of pressurized vapor to the input port 110a of sensor 110. The tube 114 is coupled at one end 114a to sensor 110 proximal to input port 110a, extends from end 114a through an aperture 112a formed in external enclosure 112, and is coupled at its other end 114b to a heated gas line 122 providing the source of heated pressurized vapor, indicated by numeral 124. Transducer assembly 100 further includes a thermal shell 116 fabricated from material having a relatively high thermal conductivity, a foil heater 118, and a control unit 120 (shown in FIG. 1A). A set of screws 121 (one of which is shown) securely mounts thermal shell 116 within enclosure 112. Foil heater 118 is wrapped around thermal shell 116, and sensor 110 is housed within thermal shell 116 so that tube 114 passes through an aperture 116a in shell 116. Control unit 120 (shown in FIG. 1A) controls the operation of heater 118, measures the capacitance across the output terminals of sensor 110 and generates therefrom a transducer output signal representative of the pressure at input port 110a.
In use, as shown in FIG. 1, end 114b of tube 114 is coupled to a heated gas line 122 containing a pressurized vapor 124 so as to provide a source of the heated, pressurized vapor 124 to input port 110a. Control unit 120 (of FIG. 1A) controls foil heater 118 so that thermal shell 116 and sensor 110 are maintained in substantial thermal equilibrium at a desired operating temperature (i.e., substantially at the same or near the desired temperature of the vapor 124 in line 122). External enclosure 112 is normally insulated from thermal shell 116 and heater 118 so that enclosure 112 normally settles at or near the ambient temperature, or room temperature, of the area surrounding enclosure 112.
Transducers such as heated transducer assembly 100 have been in use for many years, even though there are several problems associated with the transducer assembly. For example, when transducer assembly 100 is operated at relatively high temperatures, e.g., more than about 80.degree. C., tube 114 becomes a critical source of heat loss. Although during operation, the sensor 110 and the vapor 124 in line 122 are heated to substantially the same operating temperature and the temperatures of both ends 114a, 114b of tube 114 approach this operating temperature, external enclosure 112, which is typically at or near the much cooler ambient temperature, physically contacts tube 114 and tends to thermally conduct a relatively large amount of heat away from tube 114 resulting in a temperature gradient across tube 114. When the heat loss from tube 114 becomes sufficient to cause condensation of the vapor 124 in tube 114, this heat loss adversely affects the accuracy of the pressure measurement provided by transducer assembly 100. Further, even when it is not sufficient to cause condensation, the heat loss from tube 114 may establish a temperature gradient across sensor 110 thereby disturbing the thermal equilibrium of sensor 110 and adversely affecting the accuracy of the pressure measurement provided by transducer assembly 100.
In an effort to control the temperature of tube 114, users of transducer assembly 100 have applied heaters and insulation to the portion of tube 114 extending between enclosure 112 and line 122. Such measures are not always satisfactory. Even when tube 114 is heated sufficiently to prevent condensation of the vapor 124 within tube 114, external enclosure 112 still conducts a relatively large amount heat away from tube 114 and thereby establishes a temperature gradient across tube 114 and sensor 110, and as previously mentioned this temperature gradient can adversely affect the accuracy of the pressure measurement provided by transducer assembly 100.
Users of transducer assembly 100 have also enclosed the entire assembly including external enclosure 112 and tube 114 within a "thermal blanket" in an attempt to prevent condensation of gas 124 within tube 114. However, such measures may cause over heating of the control unit and thereby reduce its lifetime, and may also disturb the thermal equilibrium of sensor 110 and thereby adversely affect the accuracy of the pressure measurement provided by transducer assembly 100.
External enclosure 112 also tends to conduct a relatively large amount of heat away from thermal shell 116 via screws 121, and this heat transfer may establish a temperature gradient across shell 116. In alternative embodiments, this heat loss is controlled by eliminating screws 121 and fixing external enclosure in place by clamping it to tube 114. However, such measures merely increase the heat transfer between tube 114 and enclosure 112 and thereby exacerbate the above-described problems.
Another deficiency of transducer assembly 100 relates to the temperature sensitive nature of control unit 120 (shown in FIG. 1A). As stated above, control unit 120 normally measures the capacitance across the output terminals of sensor 110 and generates therefrom the transducer output signal which is directly representative of the pressure at input port 110a. For example, the transducer output signal is often an electrical signal characterized by a voltage that is proportional to the pressure at input port 110a, and to generate such a signal control unit 120 provides linearization as well as compensation for higher order non-linear effects associated with sensor 110. Control unit 120 also controls the operation of heater 118.
The performance of control unit 120 is generally sensitive to temperature because many of the components used to construct control unit 120 are themselves temperature sensitive. Further, the life times of many of the components used to construct control unit 120 also depend on temperature so that transducer 100 suffers from increased failure rates when control unit 120 is operated at relatively high temperatures. It is therefore desirable to maintain the operating temperature of control unit 120 at a constant so that the performance of control unit 120 does not fluctuate with changes in the ambient temperature, and it is also desirable to operate control unit 120 at a relative low temperature to decrease its failure rate.
Many prior art heated pressure transducers are designed to maintain the temperatures of the sensor 110 and of the control unit 120 at 45.degree. C., and such transducers are often referred to as "45 degree transducers". In 45 degree transducers, the control unit 120 is normally constructed from relatively inexpensive electronic components (i.e., "commercial components") that are rated to operate at the 45.degree. C. temperature. In these units, the control unit 120 is normally mounted within the external enclosure 112 and thereby forms an integral part of the transducer assembly. Due to their relatively low operating temperature, 45 degree transducers enjoy relatively low failure rates, however, their 45.degree. C. operating temperature is too low for many applications.
For many higher temperature applications, "100 degree transducers", which maintain the temperatures of their sensor 110 and their control unit 120 at 100.degree. C., are used. In 100 degree transducers, the control unit 120 is normally constructed from relatively expensive electronic components (i.e., "military components") that are rated to operate at the 100.degree. C. temperature, and the control unit is normally packaged as an integral part of the transducer 100. The use of military components increases the cost of such transducers, and even when these expensive components are used, such transducers suffer from increased failure rates do to the relatively high operating temperature.
For even higher temperature applications, "150 degree transducers", which maintain the temperatures of their sensor 110 at 150.degree. C. are used. Since the 150.degree. C. degree operating temperature is too high even for military components, the control unit 120 in such transducers is normally located remotely from sensor 110 so as to thermally isolate control unit 120 from sensor 110, and the control unit 120 is electrically coupled to sensor 110 and heater 118 by relatively long cables. However, the use of such long cables contributes noise and electrical instability to transducer assembly 100. Further, the packaging of the 150 degree transducers is inconvenient because the control unit 120 is not included as an integral part of the transducer.
Yet another deficiency of the prior art transducer assembly shown at 100 relates to its associated "start up" or "warm up" time. Transducer assembly 100 provides accurate measurements only when sensor 110 is in thermal equilibrium at the desired operating temperature, and the "warm up" time is the time required for transducer assembly 100 to transition from an initial "cold" or "room temperature" state to the desired thermal equilibrium. During warm up of transducer assembly 100, control unit 120 activates heater 118 and thereby applies heat to shell 116 in a controlled fashion so as to maintain the temperature of shell 116 at the desired operating temperature. Transducer assembly 100 includes a thermistor (which as is well known is a device having an electrical resistance that varies according to a function of the device's temperature), indicated at 164 in FIG. 1, disposed on thermal shell 116. Control unit 120 uses thermistor 164 to sense the temperature of thermal shell 116 and controls heater 118 accordingly so as to maintain the temperature of shell 116 at the desired operating temperature. Control unit 120 maintains the temperature of shell 116 equal to the desired operating temperature for as long as is required for sensor 110 to reach thermal equilibrium at the desired operating temperature. When the desired operating temperature is on the order of 150.degree. C., the warm up time of transducer assembly 100 is typically on the order of several (e.g., four) hours, and such a long "warm up" time is often inconvenient.
To facilitate maintaining sensor 110 in thermal equilibrium, transducer assembly 100 normally provides an insulating air gap between thermal shell 116 and sensor 110. Although this air gap facilitates maintaining sensor 110 in thermal equilibrium after sensor 110 has been warmed up to the desired operating temperature, this insulating air gap inconveniently increases the time required to initially warm up sensor 110.